1. Field of the Invention
The present invention relates generally to the field of plasma devices and their uses. More particularly, this invention relates to the creation and use of a microhollow cathode, cold plasma jet discharge at atmosphere.
2. Description of the Related Art
Plasma is an electrically neutral, ionized state of gas, which is composed of ions, free electrons, and neutral species. As opposed to normal gases, with plasma some or all of the electrons in the outer atomic orbits have been separated from atoms and molecules, producing ions and electrons that are no longer be bound to one other. Typically, electrical fields can be used to create plasma by accelerating (or heating) the electrons and ionizing the gas. The charged species in a plasma will interact or couple readily with electric and magnetic fields.
Practical applications of plasmas may include plasma processing, plasma displays, surface treatments, lighting, deposition, ion doping, etc. The unique properties of plasmas enable their use as a reactant for the modification of material properties. The particular response of a material depends on the composition of the gas that is ionized/energize and the method that is used to generate the plasma. For example, plasma processing has revolutionized semiconductor manufacturing processes. Plasma edging techniques provide the means for the current level of miniaturization. Plasmas have also successfully been used for the sterilization/decontamination of surfaces. Studies found that for this application, in particular, oxygen and its compounds (hydroxyl, nitric oxide) are important components of the plasma. However, until recently most studies on the decontamination efficiency of plasmas have been conducted in low pressure environments, because larger volume plasmas can be created at lower pressure.
When the ions and electrons of a plasma are at the same temperature, then the plasma is considered to be in thermal equilibrium (or a “thermal plasma.”) That is, the ions and free electrons are at a similar temperature or kinetic energy. For example, a typical thermal plasma torch used for atmospheric pressure plasma spraying may easily provide a plasma flow with temperatures between 9,000 and 13,000 K.
Non-thermal plasmas are plasmas where the electrons may be in a high state of kinetic energy or temperature, while the remaining gaseous species are at a low kinetic energy or temperature. The typical pressure for generating a non-thermal or low temperature plasma glow discharge is approximately 100 Pa. Devices that attempt to generate discharges at higher or atmospheric pressures face problems with thermal and electronic instabilities and in particular arcing within the gas and/or the electrode, sometimes leading to additional problems with electrode wear. Arcing itself constitutes the transition from a non-thermal state to a thermal plasma of high temperatures, which can damage adjacent surfaces and materials. To counteract these effects (instabilities, arcing), the linear dimension of the device may be reduced to reduce residence time of the gas in the electric field or a dielectric barrier may be inserted to separate electrodes. However, these adjustments can affect scalability and power consumption. Other cases may employ gasses intended to inhibit arcing or ionization. The field has produced few low power, atmospheric, non-thermal plasma jet capable of operating at room or near room temperature.
Some researchers have investigated the generation of non-thermal plasma discharges at atmospheric pressures. For example, a micro beam plasma generator has been described by Koinuma et al. Hideomi Koinuma et al., “Development and Application of a Microbeam Plasma Generator,” Appl. Phys. Lett. 60(7), (Feb. 17, 1992). This generator produced a micro beam plasma discharge using radio frequency (RF) and ionization of a gas that flowed between two closely spaced concentric electrodes separated by a quartz tube as a dielectric. The plasma discharge temperature was 200-400C.
Stoffels et al. has disclosed a non-thermal plasma source titled a “plasma needle.” E. Stoffels et al., “Plasma Needle: a non-destructive atmospheric plasma source for fine surface treatment of (bio)materials,” Plasma Sources Sci. Technol. 11 (2002) 383-388. The plasma needle also used an RF discharge from a metal needle; an RF electrode is mounted axially within a gas filled, grounded cylinder to generate plasma at atmospheric pressure. Plasma appeared at the tip of the needle and its corona discharge was collected by a lens and optical fiber.
Stonies et al. recently disclosed a small microwave plasma torch based on a coaxial plasma source for atmospheric pressures. Robert Stonies et al., “A new small microwave plasma torch,” Plasma Sources Sci. Technol. 13 (2004) 604-611. This torch generated a microwave induced plasma jet induced by microwaves at 2.45 GHz. Some of the features of this torch were relatively low power consumption (e.g., 20-200 W) compared to other plasma sources and its small size. However, the excitation temperature for this small plasma generator was about 4700K.
In general, micro beam generators are often limited in size by a requirement that the concentric or coaxial dielectric be limited in thickness for proper plasma generation. High pressure or atmospheric glow discharges in parallel plane electrode geometries may be prone to instabilities, particularly glow to arc transitions, and have generally been believed to be maintainable only for periods in the order of ten nanoseconds. Further, the above high pressure devices require RF or microwave signals, which can complicate practical implementation.
U.S. Pat. No. 6,262,523 to Selwyn et al. disclosed an atmospheric-pressure plasma jet with an effluent temperature no greater than 250C. This approach used planar electrodes configured such that a central flat electrode (or linear collection of rods) was sandwiched between two flat outer electrodes; gas was flowed along the plane between the electrodes while dielectric material held the electrodes in place. An RF source supplied the central electrode, which consumed 250 to 1500 W at 13.56 MHz, for an output temperature of near 100C and a flow rate of about 25-52 slpm. One function of the high flow rate is to cool the center electrode in an attempt to avoid localized emissions. This device requires Helium to limit arcing; Helium has a low Townsend coefficient so that electric discharges in Helium carry high impedance. The embodiment that employs a linear collection of rods seeks to limit arcing by creating secondary ionization within the slots between the rods, forming a form of hollow cathode effect. Although an improvement, this device requires a high flow rate of helium, along with a significant RF power input to achieve an atmospheric plasma jet near 100C.
Noble gases are generally used as the operating gas, because they facilitate the generation of stable glow discharges. In some cases, such as decontamination applications, oxygen may be mixed with the noble gas to increase efficiency; however, the level of oxygen usually does not exceed 5% of the mixture. The main reason for this low percentage is that the admixture of oxygen increases the probability of instabilities in the plasma. Conversely, the generation of a stable glow discharge in gases with higher oxygen contribution, such as air, is extremely challenging. Thus, many conventional approaches for atmospheric plasmas devices have required noble gases to avoid the uptake of admixtures of oxygen, nitrogen, and water vapor from the surrounding air, all of which would change the quality or nature of the plasma.
Another major disadvantage of conventional plasma treatment or decontamination processes, for example, is the typically very high process temperatures, which may be on the order of at least several hundred centigrade. As a consequence, most direct plasma exposures cannot be used on materials with low melting point (e.g., plastic materials), nor can they be used in many biological tissue application, such as human skin.